Realism in Duhem's Natural Classification. Pierre Duhem is an outstanding exponent of empiricism. According to the empiricist view scientific laws and theories merely describe formal relations between observable phenomena. Duhems' important notion of natural classification is intended to explain the predictive success of science. I shall argue that it can only be interpreted realistically. Besides the success of science, two further arguments are put forward in favor of realism: (i) the fact that laws of nature are necessary, and (ii) the extension of observation by using instruments